This invention relates in general to NIPI superlattices and relates more particularly to an improved type of selective contacts for a NIPI doping superlattice structure. The general theory of superlattices is presented in an article by Gottfried H. Doehler entitled SOLID-STATE SUPERLATTICES published in the November 1983 issue of Scientific American, pages 144-151. Superlattices are of interest because they have interesting electrical and optical properties, because their electrical and optical properties can be adjusted by the choice of parameters of the superlattice. Specifically, in NIPI doping superlattices, these properties can be tuned by application of electrical or electromagnetic fields.
There are two types of superlattices: a compositional superlattice (also called a heterostructure superlattice) and a doping superlattice. A compositional superlattice is a periodic array of ultrathin layers of two different semiconductors in alternation. Each layer is no more than a few hundred atoms thick so that there is significant interaction between adjacent layers. The composition of the two layers is selected so that they have a compatible lattice structure and so that the band gap in one of the layers is not equal to that in the other layer.
The effect of the superlattice structure is that the bottom of the conduction band exhibits a potential well for electrons in each of the smaller band gap layers and exhibits a potential barrier in each of the larger band gap layers. Likewise, the top of the valence band exhibits a periodic array of potential wells for holes. One such superlattice consists of an alternating sequence of layers of gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs). These potential wells break the conduction band into a series of minibands and significantly affect the electrical and optical properties of the superlattice.
A doping superlattice consists of an alternating sequence of n and p doped layers in a semiconductor. These doped layers may, but need not, be separated by layers of undoped (intrinsic) semiconductor material. The doping superlattice is also referred to as a NIPI superlattice because of the alternating n-doped, intrinsic, p-doped and intrinsic layers in such a superlattice.
The recombination of electrons from the n-type layers with holes from the p-type layers results in a periodic charge variation in the superlattice that produces a periodic variation in the bottom of the conduction band and in the top of the valence band, thereby producing a periodic array of potential wells as in a compositional superlattice. This also results in a separation between the holes and the electrons so that the recombination time for excess holes and electrons is greatly increased. When excited optically or electrically, a large number of excess holes and electrons are created that flatten the periodic potential and increase the effective band gap (defined as the distance between a minimum in the bottom of the conduction band and a maximum in the top of the valence band) of the superlattice. Therefore, the electrical and optical properties can be varied by varying the number of excess holes and electrons in the superlattice.
In order to vary the number of excess holes and electrons by electrical excitation, a pair of selective contacts need to be produced. The first selective contact needs to make a low impedance ohmic contact to the n-doped layers, but not to the p-doped layers and the second selective contact needs to make a low impedance ohmic contact to the p-doped layers, but not to the n-doped layer.
At present, selective contacts to the n- and p-type layers are formed, respectively, by depositing small tin (Sn) and tin/zinc (Sn/Zn) balls on the surface of the superlattice and then annealing the superlattice to diffuse these dopants downward into the superlattice. The diffused tin and zinc atoms cause strong n-and p-doped regions below the surface of the superlattice, thereby producing the first and second selective contacts, respectively. Unfortunately, these selective contacts are far from ideal. The sizes of the balls are variable so that reproducible results are not achieved. The deposited balls are quite large on the scale of present integrated circuit features so that the resulting contacts are equally large making this process unsuited for miniaturization. The large surface area of these contacts results in a large parasitic capacitance and also, because of band gap states at the interface between these contacts and the superlattice, results in high leakage current and high recombination rate. Extremely low electroluminescence efficiency at room temperature results because of this undesirable nonradiative recombination at the selective contacts. Therefore, it would be advantageous to develop a method that enables the production of selective contacts that are not limited by these problems.
Unfortunately, it is difficult to make a selective contact that at the same time makes a good ohmic contact with one dopant type and a good high impedance contact with the other dopant type. This difficulty is illustrated by the Sn and Sn/Zn selective contacts discussed above which produce such a high recombination rate that electroluminescence efficiency is extremely low at room temperature. Except for this one example of selective contacts, work on contacts only considered the properties of ohmic contacts to one dopant type of material and had no concern with the properties of this contact to the opposite dopant type of material.